Electronic cables for use in applications such as telecommunication are well know and provide a highway through which much of today's digital information travels. Many of the cables which transmit digital information utilize pairs of wire twisted together, i.e. “twisted pairs”, to form a balanced transmission line. One type of conventional cable for high-speed data communications includes multiple twisted pairs that are bundled and cabled together to form the high-speed cable.
Communications cable must generally achieve a high level of performance by adhering to industry standards for cable impedance, attenuation, skew and crosstalk isolation, among others. One such standard, IEEE (Institute of Electrical and Electronics Engineers) standard 802.3 for Ethernet applications, has been the key driver defining cable performance parameters and is the accepted standard for 10 gigabit per second operation.
In addition, standards exist which impose dimensional constraints and building code standards, for example fire performance safety requirements of the National Fire Protection Association (NFPA). Crosstalk is an important factor in evaluating cable performance in high tech environments as it represents signal energy loss or dissipation due to coupling between conductors or components of the cable. When twisted pairs are closely placed, electrical energy may be transferred from one pair of cable to another causing crosstalk. Such energy transfer, i.e. crosstalk, is undesirable because it causes interference to the information being transmitted through the twisted pair and can reduce the data transmission rate and can also cause an increase in the bit error rate.
Near end cross-talk (referred to as “NEXT”) occurs between twisted pairs within the same cable, causing interfere with high frequency signal transmission. To control NEXT in unshielded twisted pair (UTP) cables, many cable designs utilize extremely short lay lengths and/or a central channel filler member that acts to physically separate the twisted pairs in order to improve crosstalk performance, as described in greater detail below. It is also known to individually shield the twisted pairs (ISTP) and electrically isolate them from one another by grounding the common shield plane, as also described below.
In a conventional cable, each twisted pair has a specified distance between twists along the longitudinal direction, which is referred to as the pair lay. The direction of the twist is known as the twist direction. When adjacent twisted pairs have the same pair lay and/or twist direction, they tend to lie within a cable more closely spaced than when they have different pair lays and/or twist direction. Such close spacing may increase the amount of undesirable crosstalk which occurs between adjacent pairs. In order to reduce crosstalk between twisted pairs some conventional cables utilize a unique pair lay in order to increase the spacing between twisted pairs within the cable. The twist direction may also be varied to reduce crosstalk.
Along with varying pair lays and twist directions, individual solid metal or woven metal pair shields are sometimes used to electromagnetically isolate pairs. Although they provide improved crosstalk isolation, shielded cables are more difficult and time consuming to install and terminate. Shielded conductors are generally terminated using special tools, devices and techniques adapted for the job which can be costly. Because of the concerns with shielded cables, a popular cable currently utilized is the unshielded twisted pair (UTP) cable. Because it does not include shielded conductors, UTP is preferred by installers and plant managers, as it may be easily installed and terminated. However, conventional UTP may fail to achieve superior crosstalk isolation, as required by state of the art transmission systems, even when varying pair lays are used.
Another method utilized to reduce crosstalk is the inclusion of a separator core, for example a “+” shape divider core as disclosed in U.S. Publication 2005/0006132. Each adjacent twisted pair is separated by the legs of the divider core in order to reduce and stabilize crosstalk between the adjacent twisted pairs. In order to reduce cost and the potential fire hazard caused by the material that forms the divider core, the profiles of the cores are minimized to decrease the amount of material used in the core.
Often multiple cables are bundled together into a hybrid cable in order to provide redundant networks and also for use with multiple hook ups. By bundling multiple cables within a single unit surrounded by an outer jacket the cost of installation is reduced to the consumer as many companies charge for installation by the foot. While the above described systems help reduce crosstalk within individual cables, when multiple cables are bundled together the separator core and varying pair lays and twist directions do not reduce cross talk in adjacent cables.
Crosstalk that occurs between adjacent, bundled cables is referred to as ANEXT. Attempts have been made in the field to reduce ANEXT in addition to NEXT. For example, some cables have been wrapped or include fillers in order to make the outer surface of the cables non-cylindrical in an attempt to reduce crosstalk between adjacent cables. However, since cable installers are accustomed to cables having a circular outer circumference, such cables can result in increased labor and cost to install.
While conventional methods have been found to be generally effective for reducing crosstalk within individual cables, there is continued development in the art to reduce crosstalk between cables when multiple cables are bundled together into a hybrid cable.